In the practice of dentistry and, in particular, periodontics, planing of the root surfaces of the teeth which have been exposed because of periodontitis (pyorrhea) is an important and necessary procedure in the treatment of every periodontal patient. The degree of success clinicians have with root planing is directly proportional to the success they will have treating periodontitis. It, therefore, behooves every clinician to do a meticulous job of root planing.
Whether root planing is performed with surgical access or in conjunction with other surgical procedures or without surgical access, sterile techniques must be stringently followed in maintaining the instruments used in such procedures. Like all other surgical instruments, they must be clean and sterile and maintained that way in a sterile field, throughout the root planing procedure. Sterile conditions must be maintained because instruments come in contact with the interstitial tissue and fluids of the periodontium and could introduce microorganisms or foreign bodies into the healthy tissues of the body.
The specific hand instruments, which are universally used, for root planing are the periodontal curettes. These instruments are made of stainless steel, so they will not rust or tarnish when they are subjected to cleaning and sterilization procedures. Instruments must stand up to autoclaving which exposes them to a temperature of 250 degrees F. at 18 pounds of vapor pressure per square inch for twenty minutes.
The two main objectives of root planing teeth (hard tissue curettage) are to remove all cementum and dentin which have been altered by the permeation of endotoxins and other toxic substances, and, second, to create a root surface which is hard and smooth.
In order to achieve the above goals in a meticulous manner, some degree of controlled penetration into the hard cementum or dentin is necessary. This controlled penetration can only be accomplished with curettes that are meticulously sharp and kept that way throughout the procedure. A dull instrument will not effectively remove the toxin-impregnated cementum or dentin and will not produce a smooth, hard surface.
A rough sharpened instrument will remove too much healthy cementum and dentin and not leave a smooth surface. A smooth surface is necessary for maintenance of plaque control. The rough tooth surface has a greater root surface per unit of area. A rough surface is difficult to maintain for plaque control.
Periodontal curettes become dull during the root planing procedure. In order to keep a sharp edge on the instruments during root planing, without contaminating them, a sterile sharpening device must be kept in the sterile pack, along with the sterile instruments.
When periodontal instruments are sharpened, the steel surface is drawn across a stone and bits of metal are abraded away. Serrations are formed on the surface of the instrument. Serrations on two intersecting surfaces of a curette form points on the cutting edge. An objective in sharpening a curette is to minimize the depth of the serrations on the intersecting surfaces, to minimize the size of the points formed on the cutting edge. For best results, the serrations (ridges and gullies) should run parallel to each other and perpendicular to the cutting edge. The serrations on one of the surfaces should not run parallel to the cutting edge, because this weakens the edge. For maximum strength of the edge, the base should not be decreased in order to maintain the base to point ratio. When the base is reduced, the points break off easier and further down the base. This means a duller instrument faster. A more desirable objective is to sharpen the curette with no formation of serrations or with serrations which have very shallow gullies.
The results obtained while sharpening instruments depend upon the compatibility of the characteristics of the sharpening stone with the type of metal alloy in the instrument. The alloys used in periodontal instruments are all about the same composition. The metal is stainless steel. Conventional sharpening stones, commonly used in root planing, are made up of abrasive particles which do the abrading and a bonding agent that holds the abrasive particles together. The characteristics of conventional sharpening stones depend on:
(1) Type and hardness of the abrasive particle. PA0 (2) Size, shape and uniformity of the abrasive particle. PA0 (3) Height of the abrasive particle above the bonding agent on the surface of the stone. PA0 (4) The distance between the abrasive particles on the surface of the stone. PA0 (5) The firmness of the bonding agent holding the abrasive particles together.
Varying these different characteristics in a stone will modify the results produced in the sharpening process. For example, stones with small-size abrasive particles spaced close together will produce smaller serrations than stones of the same abrasive particle size whose abrasive particles are spaced farther apart. Sharpening stones with the abrasive particles closer together will generate far more heat from friction, and it follows that less heat is generated when the abrasive particles are spaced farther apart. This is one of the reasons, even with a coolant, mechanical sharpeners cannot produce a meticulously sharp edge without overheating the metal in the instrument. This overheating removes the temper from the metal and, with it, its ability to hold a edge.
Conventional sharpening stones with weaker bonding agents give up the abrasive particles freely in order to abrade to less serration depth and less heat from friction, but these stones require a lubricant. The lubricant acts as a vehicle for the abrasive particles to come to the surface of the stone, not to make the abrasive particle slippery to reduce abrasion, thus friction. It is the abrasion that does the sharpening. If a lubricant is not used with these types of stones, deeper serrations in the metal surface will result.
Sharpening stones are either man-made or found in nature. The common man-made stones used in periodontics have either aluminum oxide or silicone carbide as the abrasive particle. These abrasive particles are used in different grit sizes and in different concentrations, with different types of bonding agents. The stone found in nature, commonly used in dentistry, has naviculite as the abrasive particle. This is the Arkansas stone.
None of the sharpening stones commonly used to sharpen periodontal instruments were designed and manufactured specifically for periodontal instruments. These are some of the problems with these conventional stones:
(1) None of the stones will produce a meticulously sharp edge (an edge produced by surfaces that do not show their serration under a ten power eye loupe).
(2) The Arkansas stone will produce a fairly good edge, but a lubricant must be used. Abrasive particles are given up in the sharpening process which grooves and valleys on the sharpening surface. The Arkansas stone is porous and difficult to scrub clean and will not hold up during sterilization in the autoclave.
(3) Some stones that will hold up under autoclaving are far too rough for good results. The stones usually need a lubricant and they usually give up abrasive particles during sharpening. The giving up of abrasive particles not only causes grooves on the surface of the stone but the particles given up can mistakenly be transferred to the periodontium as a foreign body. Such foreign bodies can cause a foreign body reaction by the patient.
(4) Common stones all remove too much metal from the instrument in the sharpening process and, therefore, reduce the life of the instrument.
When sharpening with the coarse conventional stones, the curettes end up with a cutting edge made up of large points, that is in essence a rake. When such a working edge is tested on a fingernail, it grabs and this is mistaken for being sharp. This rake edge produces a rough surface on the root of the tooth and too much good tooth structure is removed. When the curette is used to root plane, the metal points break off because the points lack support from the base. This rake edge lacks cutting ability, it must tear through the cementum and dentin leaving a rough surface. This dullness becomes aggravated with use. Each of the non-cutting, broken points becomes a stress point. The shock on the points being advanced through cementum and dentin, tends to break off the points further down towards the base, thus breaking down the edge still further. This means the rake edge breaks down faster and farther and requires that much more metal be removed in order to resharpen the curette. Also the sharpening must be repeated more often.
Consequently, coarse stones reduce the life of the instrument. This is why a meticulously fine edge, maintained on a instrument during root planing, increases the life of the instrument.
Clinicians typically believe that the curettes they purchase new, come from the manufacturer properly sharpened for root planing. One can almost see with the naked eye (more dramatically observed with a jewelers loupe), that two things are wrong with the blade of the instrument. First, there are deep serrations on the face and side of the instrument that forms the working edge. Second, the serrations on the face of the instrument run parallel or nearly parallel to the working edge. The reasons for such serrations are that the curettes were sharpened at the factory with a mechanical sharpener. Even though manufacturers use a coolant, they have to use a coarse stone with the abrasive particles spaced far apart or they will overheat the metal during sharpening and all the temper of the steel will be lost and, with it, the ability to hold a sharp edge.
Further, mechanical grinding wheels cannot sharpen the face of a curette in a direction perpendicular to the cutting edge. Consequently, a clinician must spend substantial time sharpening a new instrument and correcting these defects.
Clinicians have had to accept the difficulty of working with instruments that have been sharpened with a coarse stone. The roughness in the cutting edge of the instrument is replicated on the root surface during the root planing procedure, thus a rough surface is produced. With the rake edge, the cutting edge removes an excess of good sound tooth structure. Also, with a rake edge, more hand pressure on the instrument is required, and more time is needed to root plan the teeth. This means a clinician has less tactile sense and requires more time with more trauma to the patient.